[OpenJDK 2D-Dev] More incompatibilities

[Jim Graham]

I was aware that the code was playing with overflow fire when I ported 
it a while back.  I figured the real fix was to eventually switch to 
float instead, as per your last suggestion - at least for the line 
widening code.  Any takers?

			...jim

Roman Kennke wrote:
> Hi there,
> 
>> 4. StrokeShapeTest: createStrokedShape() behaves differently.
> 
> It turns out that there is an arithmetic overflow here. The pisces
> stroker does a stupid thing here. First it initializes the
> scaledLineWidth like this:
> 
>         this.scaledLineWidth2 = ((long)transform.m00*lineWidth2);
> 
> which is infact wrong, because in fixed point arithmetics you need to
> apply >> 16, because the decimal moves.
> 
> However, in another place it uses this factor like this:
> 
>                 dx = (int)( (ly*scaledLineWidth2)/ilen >> 16);
>                 dy = (int)(-(lx*scaledLineWidth2)/ilen >> 16);
> 
> which makes it ok in the end. The only problem is, that we start with an
> incredibly large number, then multiply another incredibly large number
> and _after_ that (when the overflow already occured) do the >> 16. The
> patch moves the >> 16 to the initialization of scaledLineWidth, which
> makes it clearer, more correct and even a tad faster, because we do the
> second calculation at least 2x.
> 
> Of course, the real fix here would be to turn the whole implementation
> into floating point. This particular testcase is fixed by this patch,
> and the 'range of correct behaviour' is much large now, but if you deal
> with very large numbers in your shapes, then you will get trouble again.
> 
> /Roman
>